1. Field of the Invention
The present invention relates to a thermal transfer printing method in which an ink sheet including thermally transferable solid ink layers formed on a base film is used and the above described solid ink layers are heated by a thermal head and transferred onto a record medium so that a picture to be printed is decomposed into a plurality of picture elements to perform printing.
2. Description of the Prior Art
A thermal transfer printing method, as well as an ink jet printing method, has become noticed as one of the simple methods for printing on plain paper and for color printing, and recently has begun to be practically applied. As the thermal transfer method, two methods are known: one is a "melting transfer method" in which solid ink layers provided on a base film are heated and melted to be attached and printed onto plain paper, and the other is a "sublimation transfer method" in which sublimating dyes included in ink layers are sublimated or evaporated.
FIG. 1 is a schematic view showing a basic structure of a conventional melting transfer printing apparatus. A solid ink layer 2 having thermal transferability is provided on a base film 1 so that an ink sheet 3 is formed. The base film 1 is made of condenser paper, for example. The solid ink layer 2 is made of wax or organic resin binder and coloring agents. A thermal head 5 is provided on the side of the base film 1 of the ink sheet 3. A rubber roller 7 is provided on the side of the solid ink layer 2 of the ink sheet 3. Between the solid ink layer 2 and the rubber roller 7, plain paper 4 is provided. The thermal head 5 includes a plurality of heating resistors 6 in a dotted configuration, for example. When the heating resistors 6 of the thermal head 5 are supplied with electric current and heated, the solid ink layer 2 is heated and melted through the base film 1 of the ink sheet 3 and a melted portion 2A is transferred onto the plain paper 4. If either the magnitude (W/dot) of the voltage applied to the heating resistors 6 or the pulse width is changed, in principle the amount of the solid ink layer 2A to be transferred changes and it seems possible to perform gradation printing.
FIG. 2 is a graph showing a relation between the power applied to the thermal head and the optical reflection density OD of a printed picture element. In this case, the thermal head is one for use in facsimile and the size of the heating resistor is 80.times.200 .mu.m.sup.2. Referring to FIG. 2, the circles indicate averages of the values measured ten times and each bar indicates a range of scatter of the measured values. The pulse width of the heating current is 2 m sec. As is clear from the graph, the amount of scatter in measured values of the optical reflection density OD is extremely large when a picture element of an intermediate optical density is printed using a conventional ink sheet. As a result, using of the ink sheet which serves for melting transfer operation contains a disadvantage that a picture including picture elements whose optical densities are changed, that is, a gradation picture cannot be stably printed.
FIG. 3 is a schematic view showing a structure of a conventional ink sheet whereby three-value picture element optical densities can be obtained. A base film 1 is coated with a black solid ink layer 8 having a high melting point, which is further coated with a gray solid ink layer 9 having a low melting point. Thus, an ink sheet 10 for three-value printing is formed.
FIG. 4 is a conception view showing a state in which solid ink layers are transferred from the FIG. 3 ink sheet onto the plain paper 4. FIG. 4A shows a state in which only the gray solid ink layer 9 is transferred onto the plain paper 4. This transfer in FIG. 4A can be performed by selecting at temperature at which the gray solid ink layer 9 is melted but the black solid ink layer 8 is not melted. FIG. 4B shows a state in which both the black solid ink layer 8 and the gray solid ink layer 9 are transferred onto the plain paper 4. The transfer in this case can be performed by raising the temperature to a point at which the black solid ink layer 8 and the gray solid ink layer 9 are both melted. Thus, using the ink sheet 10 for three-value printing shown in FIG. 3, three-value optical densities for picture elements, that is, two optical densities shown in FIGS. 4A and 4B and the optical density in case where no transfer is made, can be obtained.
In theory, an ink sheet of more than three values can be formed if a large number of solid ink layers having different melting points are provided in an overlapping manner on the base film 1, but practically three values are the maximum limit for this kind of ink sheet. This is because on the low temperature side, a picture to be printed cannot be well fixed to the plain paper, and on the high temperature side, the heat resisting property of the thermal head is limited. In an ink sheet for three-value printing as described above, variation of the ambient temperature or variation of the heating temperature of a heating material due to repetition of printing operation, etc. increases instability such as blackening of a picture element to be colored gray. In addition, since two solid ink layers are superimposed on the base film, there is a disadvantage in that it is difficult to manufacture an ink sheet.
As a result, it is desired to provide a thermal transfer printing method capable of printing the elements in the gradation, stably and with excellent reproducibility.
A new method for thermal transfer printing is described in the copending U.S. patent application Ser. No. 494,283 now U.S. Pat. No. 4,492,965 filed May 13, 1983 by the inventors of the present application. The present invention makes an improvement of the method of this copending application and provides a thermal head to be employed in the method.